The invention concerns a sensor for measuring the speed and/or position of a piston in relation to that of the cylinder it moves inside of in a dashpot or shock absorber in a motor vehicle.
Dashpots or shock absorbers are employed as linkages that transmit vibrations from the roadway to the body by way of the wheel-suspension system. Shock absorbers are mainly employed at the front axle of the motor vehicle. The mechanisms are often provided with active or semi-active vibration-suppression controls to ensure passenger comfort and safety.
Such controls require measuring various variables of motion and transmitting them to the controls. These variables include the speed of the body in relation to that of the axle, the position of the piston in the cylinder, or the acceleration of the body. The controls convert them into parameters for varying the force that suppresses the vibrations.
German Patent 3 909 190 describes a relative-speed sensor for a dashpot. The sensor comprises a cylindrical coil accommodated in a jacket connected to the piston rod and a permanent magnet at the head of the cylinder. The sensor supplies a relative-speed signal derived from the voltage induced in the winding. It is impossible to measure and accordingly control how far inside or outside the piston is.
A sensor that operates precisely inside a dashpot and comprises a spool in the jacket and a magnet in the cylinder's bearing seal also has another drawback. Employed in a shock absorber, its results would be so imprecise as to render the absorber impossible to control. A shock absorber has a helical spring that rests against a disk in a tubular housing. The spring acts as an additional coil. The motion of the helical spring in relation to the sensor winding and the varying separation between the spring's windings generates inductive interference that the system processor cannot compensate for.
Again, shock absorbers often take up so much space that they have to be protected by rubber bellows instead of jackets. Such bellows prevent the use of a state-of-the-art sensor winding.